During avian vocal learning, birds memorize conspecific songs and use auditory feedback to shape their vocalizations to these stored models. Often song learning is restricted to species-specific sensitive periods, and this proposal is aimed at identifying neural changes that may encourage and temporally constrain this learning. The general approach is to describe how relevant brain regions normally change during vocal learning, and then to assess the functional significance of these cellular changes by examining how they are influenced by auditory inputs essential to song learning. The studies concern zebra finches (Poephilla guttata), a species whose song development is associated with major anatomical and neurochemical changes within brain pathways controlling song. The studies proposed examine two specific neural changes that accompany song learning; a massive addition of new neurons to the vocal motor path way, and a decline in the N- methyl -D- aspartate class of excitatory amino acid receptor within a region implicated in song development. In the first set of studies, thymidine autoradiography and retrograde tracing will be combined to assess how auditory inputs during either song memorization or vocal practice influence the survival of new neurons inserted into the vocal motor pathway. Also, long-lasting retrograde tracers will be used to determine if neuronal replacement or addition occurs within this same neural pathway in adulthood, when song patterns are stable. In the second experimental series, neurotransmitter receptor autoradiography will be used to determine if: i) the developmental regulation of NMDA receptor binding correlates with age-related differences in the propensity for vocal learning and, 2) if auditory manipulations that delay completion of either song memorization or vocal practice similarly affect the timecourse of NMDA receptor decline. This receptor subtype has been linked to numerous other instances of experience-dependent developmental plasticity and learning. Sensitive periods are characteristic of a wide variety of behavioral phenomenon (e.g. language acquisition, social attachment, imprinting) and these studies will help define neural mechanisms that encourage learning during these periods of unique susceptibility.